The present disclosure is related to a radiation image processing apparatus, a radiation image processing method, and a radiation image processing program that generates subtraction images from tomographic images obtained by tomosynthesis imaging under different imaging conditions.
Recently, tomosynthesis imaging has been proposed for radiation image obtaining apparatuses that employ radiation such as X rays and gamma rays. Tomosynthesis imaging obtains images by irradiating radiation onto a subject from a plurality of radiation source positions by moving a radiation source, and generates a tomographic image in which a desired cross sectional plane is emphasized, from a plurality of projection images obtained by the imaging operations, in order to observe an afflicted portion in greater detail. In tomosynthesis imaging, the radiation source may be moved parallel to a radiation detector or moved along a circular or elliptical trajectory according to the characteristics of an imaging apparatus or necessary tomographic images. The subject is imaged at a plurality of radiation source images to obtain a plurality of projection images. A tomographic image is generated by reconstructing the projection images by an reconstruction method such as the shift and add method, the simple back projection method or the FBP method (Filtered Back Projection method).
Here, the FBP method is a reconstruction method that designates plane parallel cross sectional scanning of tomosynthesis imaging as a portion of a cone beam CT scan, and expands the filtered back projection method. If a projection image is merely back projected, noise will occur in the periphery of a coordinate position which is a target of processing. Therefore, the FBP method back projects a projection image after applying a filter thereon, to reduce blurring in images that flows in the direction of movement of the radiation source, and obtains an image closer to the intended target.
Meanwhile, there is a problem in tomosynthesis imaging that reconstructed tomographic images become blurred due to the influence of mechanical errors of an imaging apparatus, body movement of a subject caused by temporal differences among imaging operations at each of a plurality of radiation source positions, etc. If a tomographic image is blurred, it will become difficult for lesions such as fine calcifications, which are effective in early diagnosis of breast cancer, to be detected, in the case that the subject is a breast. For this reason, simple imaging is also performed when performing tomosynthesis imaging, to obtain both tomographic images, and two dimensional images. For example, radiation imaging apparatuses for imaging breasts (called mammography apparatuses) that perform both tomosynthesis imaging and simple imaging with breasts maintained in a compressed state have been proposed (refer to PCT Japanese Publication No. 2014-507250 and Japanese Unexamined Patent Publication No. 2012-166026).
In addition, subtraction processes are performed in mammography, in order to facilitate discrimination of lesions. Here, subtraction processes refer to a process in which an image corresponding to a difference in a plurality of radiation images imaged under different imaging conditions is obtained. Specifically, a subtraction process is administered for each corresponding pixel within the plurality of images, to emphasize or extract a specific portion of the subject within the radiation images, that is, to obtain a subtraction image.
There are basically two types of subtraction processes which are performed in mammography. One type of subtraction process is temporal subtraction. In temporal subtraction, an image, in which blood vessels of a breast are not emphasized, is subtracted from an image, in which the blood vessels of the breast are emphasized by imaging after injecting a contrast agent into the breast from a vein, to extract the blood vessel portions. The other type of subtraction process is energy subtraction. Energy subtraction utilizes the fact that contrast agents have different radiation absorption rates with respect to radiation having different energies. In energy subtraction, radiation having different energies are irradiated onto a breast after a contrast agent is injected, to obtain radiation images having different energies. The plurality of radiation images are weighted appropriately and differences among the radiation images are calculated, to extract the blood vessel portions of the breast. In addition, there are cases in which radiation is irradiated onto a breast at temporal intervals after a contrast agent is injected to obtain a plurality of radiation images. In such cases, the plurality of radiation images are weighted appropriately and differences among the radiation images are calculated, to ascertain the spread of the contrast agent through the blood vessel portions of the breast.
In many cases, breast cancer progresses by the blood vessel walls being weak and newly formed blood vessels that spread randomly toward the periphery increasing. The increased newly formed blood vessels increase transmissivity and are serpentine, different from normal blood vessels. Accordingly, the increased newly formed blood vessels can be extracted by employing subtraction images, thereby facilitating detection of breast cancer.
For this reason, a technique in which two radiation images to be subjected to a subtraction process are obtained to generate a subtraction image in addition to obtaining tomographic images by tomosynthesis imaging, and the subtraction image and the tomographic images are displayed has been proposed (refer to PCT Japanese Publication No. 2014-507250). In addition, a CE-DBT (Contrast Enhanced Digital Breast Tomosynthesis) technique that obtains radiation images to be subjected to subtraction processes at each radiation source position when performing tomosynthesis imaging has also been proposed (refer to Japanese Unexamined Patent Publication No. 2012-166026). According to the technique disclosed in Japanese Unexamined Patent Publication No. 2012-166026, a plurality of radiation images are obtained by irradiating radiation having different energies onto a subject while performing tomosynthesis imaging, a plurality of tomographic images having different energies are generated from the obtained plurality of radiation images, and a subtraction process is administered onto the tomographic images, to obtain subtraction images of the tomographic images (hereinafter, referred to as “tomographic subtraction images”).